I just got the Metabones EF-E MkIII adaptor. It works great on the A7R. But does anybody know what is and how to use that "wide open button" on the adaptor?

On the website it says it's opposite to a dof preview button. But isn't the lens is always wide open anyway? Is it for movie mode (I need to play with that to see)?

...all the options pretty quickly:

there are two modes in which aperture control behaves differently: the "green" mode (available when we mount the adapter while depressing the DOF button), and the "normal" mode (with mounting the adapter without pressing the DOF button);

but also it depends too on whether the option LV Display --> setting effect is ON or OFF;

depending on the above we can have aperture set wide open all the time (Canon style) with closing the iris only on the full-shutter-press, or aperture set for a preview (Sony style) and being opened only by DOF-button press, or... 1/2-press of the shutter button. Just try it in all combinations and you'll see how it works,